A trap is a bend (with or without an enlargement) of the pipe, intended to retain a sufficient amount of the fluids that enter it to occlude the pipe and prevent the backward flow of air. The effective water-seal is represented by the total depth of water in the trap, minus the inside diameter of the pipe, and should be at least one inch.
Fig. 2.—Offset. Fig. 3.—S-trap. Fig. 4.—Half S-trap.
Traps are of various kinds, and many are patented. The accompanying figures represent the principal types.
Fig. 5.—D-trap. Fig. 6.—Bag-trap.
Fig. 3 is the S-trap. Fig. 4 is the half S-trap. There is also a three-quarter S-trap. Fig. 5 is a D-trap, formerly much used, but now mostly out of date, because it retains dirt. Fig. 6 is a bag-trap. It has no advantage over the S-trap, and uses more material. Fig. 7 is a bottle-trap, much used in Boston. This also retains dirt. There are varieties of it, the distinguishing feature of all being that the fluids enter them at the bottom and flow out at the top.
Fig. 7.—Bottle-trap. Fig. 8.—Bell-trap.
